Phantom Power and Microphone Splitters

There are several ways to split a microphone's output. They range from cheap (wye-cord or paralleled inputs) to expensive (BSS active mic splitter). Somewhere in the middle there is the venerable old transformer coupled splitter.

For sure, the active splitter is the best solution. The microphone only sees one load, which is the splitter's input impedance. Each of the outputs is buffered from the rest. The only rub is that the splitter's sonics are imposed on everyone that takes one of the split outputs.

In this case, the splitter supplies phantom power to the microphone and could care less what the mixers are doing.

Transformers are the next-best solution. Unfortunately the usual 1:1:1 transformer still parallels the input impedances of the two consoles. From a microphone loading perspective, this is no better than simply paralleling the mixer inputs. If you run into a nasty grounding situation, the transformer coupled splitter will save your bacon. For phantom power purposes, what usually happens is that the FOH mixer gets a direct feed from the microphone and the monitor mixer gets a transformer-isolated split (in reality, a parallel feed) from the mic. The FOH mixer supplies phantom and everyone is happy. In a really nasty scene, both mixers might get transformer splits, and the splitter itself has phantom feed resistors and a 48V DC source.

In many instances, simply connecting the mixers in parallel works just fine. You can get into trouble if you aren't really careful about how things get grounded, especially with regard to the FOH console's ground system and the monitor console's ground system. If either of the boards has pin-1 trouble, this may be exacerbated. I've used this method myself for over 30 years and only occasionally had problems. In each instance, I can trace the source of trouble back to not following the rules. The big rub with parallel connected mixers is the old question: "Who supplies phantom?"

Some consoles don't like having phantom power applied via their microphone inputs when their internal supply is not switched on.

This is especially true of mixers that don't have individual phantom on/off switches or at least switches for groups of inputs. What happens here is that the manufacturer took the easy way out to turn the phantom power supply on and off: they switched the AC feed to the phantom supply rectifier. The voltage regulator that supplies 48V to all of the phantom feed resistors is always connected. When the mixer that isn't supplying phantom has it's inputs 'backwashed' with phantom power, the regulator doesn't quite know how to deal with this voltage coming at it from what it thought was its output.

The cure here is a simple series diode connected at the output of the regulator, and a large electrolytic capacitor to ensure that the output impedance of the regulator stays low in spite of the series diode.

The Mackie SR series mixers that I had a hand in have this series diode. Other mixers may, and it pays to check the schematic for this feature.

Now, having said that, let's explore the situation when both mixers supply phantom.

Quite simply, we have two 48V supplies driving the mic through 6800 ohm resistors on each leg. As far as the microphone is concerned, it's no different than if we halved the value of the feed resistors to 3400 ohms each. So, again from the microphone's perspective, the mic is seeing 48V thru 3400 ohms in parallel with 3400 ohms. This means that twice the current is available to every microphone. Looking inside the microphone, we typically have a zener diode with or without some extra current limiting resistance. So, the effect of the extra current is that the zener diode works a bit harder.

I have used sound systems where both the monitor and FOH boards supplied phantom powering to the microphones. I've done this with just about everything from cheap electrets to Neumanns and AKGs. It works, and the microphones have all said, "so what." The Neumanns are especially immune to this. The 9-52V AKGs (old C451's, C414's) have to work their zeners a bit harder, but it's no big deal. They're a worst case example. My C451's and C414's still work just fine, thank you.

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Copyright © 1999-2009 by Rick Chinn. All rights reserved.

Last modified 04/14/2011. 18:27:08